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Class D feedback and stability
Class D feedback and stability
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Old 25th September 2018, 11:42 AM   #1
TroelsM is offline TroelsM  Denmark
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Default Class D feedback and stability

Hi Guys

I've been searching for this and cannot find it.

For all of us that dont hold a PhD in system engineering and control theory, it is a bit hard to distinguish the different class D typologies and feedback-schemes. - and especially what pros and cons the different schemes have.

So far I know and (sort of... ) understand the difference between the clocked and self-osc designs. But within the self-osc group we have post- and pre-filter feedback, first and second order integrator, UcD and much more.

Does anyone know of reference/doc/pdf with an (simple, non-math-heavy) overview?

I have build and simulated a smallish classD (feedback-scheme like iraudamp) and I get some OK results, but then I got a bit more brave and started testing clipping behavior, squarewave-response, no-load, and different start-up scenarios. - Thats when things start to fall apart...

Any inputs are welcome. I can share schematic and results later.

Attachment: first, single-sided prototype: IR2184, IRF640N, +/-50V, 82% eff, 0.1% dist. Not impressive, and yes the 2184 is not a prime candidate for this.

Kind regards TroelsM
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Last edited by TroelsM; 25th September 2018 at 11:49 AM.
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Old 25th September 2018, 08:13 PM   #2
FauxFrench is offline FauxFrench  France
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Hej Troels,

I noticed you did not get much response on your question. It is not because it is a trivial question, on the contrary it is a very relevant and challenging query.
Most of us use class D amplifier chips more or less according to the datasheet and application notes. Then, we do not need to challenge our brains with this complex issue. I notice, only few manufacturers recommend post filter feedback. The 30-60 KHz output filter cut-off frequency will limit the feedback effect. It doesn't mean post-filter feedback should be discarded without reconsideration.

The reason why you do not have such detailed descriptions of how to make successful feedback loops for class D amplifiers is probably that this knowledge is the lifeblood of the manufacturers.
All know today how to design a good output push-pull stage and the corresponding driver stage. The only real difference in sound can be implemented by a better analog→PWM conversion with the right feed-back (or eventually feed-forward) to achieve that. The manufacturers are most likely not going to tell their competitors how they do.
There is one source of information that may be useful – patents. To obtain a patent, you need to disclose your idea. But, a patent disclosure is often not so easy to understand for practical use. I once glanced at what is supposed to be one of the central patents for “class T” modulation (Dr. Triphati, founder of Tripath). It was about relevant DSP and I could not use that information for much. Perhaps other patents are easier to understand. Analog feedback (or -forward) is quite easy to understand qualitatively. DSP very quickly gets mathematical and a PhD is useful.

On UcD, Philips or Hypex may have some information. It may not be impartial (balanced) information as they have own economical interests.

If you will disclose your findings on the topic, I am looking forward to learn.

FF

Last edited by FauxFrench; 25th September 2018 at 08:21 PM.
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Old 25th September 2018, 08:57 PM   #3
Reactance is offline Reactance  South Africa
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Quote:
Originally Posted by TroelsM View Post
Hi Guys

I've been searching for this and cannot find it.

For all of us that dont hold a PhD in system engineering and control theory, it is a bit hard to distinguish the different class D typologies and feedback-schemes. - and especially what pros and cons the different schemes have.

So far I know and (sort of... ) understand the difference between the clocked and self-osc designs. But within the self-osc group we have post- and pre-filter feedback, first and second order integrator, UcD and much more.

Does anyone know of reference/doc/pdf with an (simple, non-math-heavy) overview?

I have build and simulated a smallish classD (feedback-scheme like iraudamp) and I get some OK results, but then I got a bit more brave and started testing clipping behavior, squarewave-response, no-load, and different start-up scenarios. - Thats when things start to fall apart...

Any inputs are welcome. I can share schematic and results later.

Attachment: first, single-sided prototype: IR2184, IRF640N, +/-50V, 82% eff, 0.1% dist. Not impressive, and yes the 2184 is not a prime candidate for this.

Kind regards TroelsM
The fact that you have a prototype is a step in the right direction, I'm keen to learn more in this area but don't have the time, however I suggest you look at Chocoholic's Class-Dlite amplifier and read the early stage development of the thread (lots of neat tips and tricks) specifically around the modulator not too detailed but enough to get you somewhere, there you will find what few have done here, build an amplifier from start to finish with serious R&D and help from others (and out performs the iraudamp), forum members phase_accurate (Charles Lehman) has supplied hes approach to the modulator scheme.

see attachment for the simple post feedback topology with some examples on the calculations you maybe need for your research.

Quote:

"
A feedback topology shall be proposed that allows the straightforward inclusion of the output filter into the feedback loop. It can be adapted to high- or low- Q output filters equally well. It is not restricted to either self-oscillating or carrier – based modulator topologies. One advantage is that the slew-rate requirements on the operational-amplifiers used for the proposed topology are quite relaxed since they are not forced to handle fast transients. Another advantage is that the feedback branch is based on resistors only.
"
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Last edited by Reactance; 25th September 2018 at 09:00 PM.
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Old 26th September 2018, 07:45 AM   #4
TroelsM is offline TroelsM  Denmark
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Thanks for the feedback. That PDF may be a good starting point.

TroelsM
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Old 26th September 2018, 08:11 AM   #5
kartino is offline kartino  Indonesia
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Good feedback = less load dependant
From there then of course post filter feedback is better
But why less design use it?
The answer is because it need very good design compared with prefilter feedback because post filter feedback deals with a mV signal comparator which is not easy to create a good shape of triangle signal at very low voltage.

Post filter is straight forward take square output changed to triangle which can be very easy to get a good triangle shape. This is the good advantage. But it has less achurate because this feedback does not include LPF filter.

I am not PHD and this is just my opinion.
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Old 28th September 2018, 06:32 AM   #6
luka is offline luka  Slovenia
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Class D feedback and stability
I think there is a lot of information scattered on this forum alone, but to remember where each information was is difficult to say the least. Chocoholic bigger amp, systemD_2k4 have lot of information. but in general, first you select your "type", how amp will work. Here you have clocked, ucd, hysteresis,... design. For each you need to know, what their +'s and -'s are.
But in the end, the single most important thing is feedback. If you have perfect or imperfect pcb layout, feedback will have to do all the work. But even feedback can do only so much, if stages before it are sloppy. Feedback can do only so much.

I've seen many things can be done, but are far above average diy user, let that be clocked, ucd, hysteresis or any other. Some people look at waveforms and say, what do I need to do, to improve the design. Like have multi-slope ramp to which you compare your sine wave. This will impact how amp will work above some level of signal (line near clipping).

But as I see it, if you would have perfect layout or no component, pcb, wire inductances, capacitances and fets would be perfect switches and there was no input to output delays (or be constant), then your only worry would be (PID) feedback. But even feedbacks are complex, there is not one for everything, you can get lost in only this topic alone
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Old 28th September 2018, 09:07 AM   #7
Reactance is offline Reactance  South Africa
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Luke has supplied some interesting insight.

I have read hypex uses a type-5 compensation in their design, bruno has confirmed this, the frequency response of the modulator can handle up to 5Mhz of bandwidth.
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Old 28th September 2018, 12:20 PM   #8
luka is offline luka  Slovenia
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Class D feedback and stability
I would also recommend to take 50min to listen to:
YouTube

Ncore amp, as it was said in this video, needs only ~200kHz worth of bandwidth, to have 53dB of gain in feedback loop @ 20kHz, in other terms, this would the as much control as ~9MHz bandwidth linear amp

Now am not sure, but does Ncore have two output inductors?
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Last edited by luka; 28th September 2018 at 12:36 PM.
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Old 28th September 2018, 04:56 PM   #9
Reactance is offline Reactance  South Africa
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Quote:
Originally Posted by luka View Post
I would also recommend to take 50min to listen to:
YouTube

Ncore amp, as it was said in this video, needs only ~200kHz worth of bandwidth, to have 53dB of gain in feedback loop @ 20kHz, in other terms, this would the as much control as ~9MHz bandwidth linear amp

Now am not sure, but does Ncore have two output inductors?
wow
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Old 3rd October 2018, 11:21 AM   #10
TroelsM is offline TroelsM  Denmark
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OK, lunchbreak update:

I have semi-OK results with the "standard" pre-filter feedback. Distortion is high (Thd:0.05, thd-noise:0.15), but it appears to be stable: no start-up problems, clipping is semi-clean. -but I wanted some post-filter feedback to tame the output if the load is removed. Also as a purely intellectual exercise for my foggy brain.

I did not have much luck with post-filter feedback and either it would have start-up problems or behave badly to no-load or clipping. Most likely due to my lack of knowledge, but it didn't work for me.

I read somewhere that a company use the output-cap-current as feedback. Without much theory I put together a rough simulation. I thought it was less than ideal to sense the actual cap-currrent so instead I put a smaller cap in parallel and sensed the current with a 100mOhm series-resistor. The current in the 2 caps should be the same with gain-difference?

It did not behave very well until i split the org feedback resistor in two and put a cap over the bigger half ( someone: that must have a fancy name in regulation-theory). Anywhos. I guess the cap lowers gain at high freq?

in LTSpice it really helps to dampen the unloaded filter-response. No impact on distortion (approx 70dB down).

I do not claim to be the first to do this, but how wacky is it and what are the downsides?

Rough sketch attached

Kind regards TroelsM
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